downloading daisy zip book, transfer to VR stream

It's unfortunately not documented by Humanware, but the process is easy, and I've been doing it for about 6 years now.
First, you do need to get the "user keys" from Humanware and install them on your Stream per their instructions if you want to read Talking Book material downloaded from the BARD website. The basic process is to format your SD card using the Stream, copy the user keys Humanware has emailed you to this SD card, then restarting your Stream with the SD card inserted. As I recall, the keys automatically unpack and install on your Stream without you having to do anything else.
Note that you should format the SD card on your Stream before putting it into your Mac's SD card reader. Formatting on the Stream ensures it has the required form for the Stream to read it, and also lets the Stream place half a dozen or so folders/subdirectories on the card where you will copy your books to. I've never tried formatting it on my Mac instead, but I'm as assuming things won't go well if you try it, so just format the thing on the Stream.
Next, go to the BARD website and download whatever books you wish to read. The Mac will download the book, unzip it, and delete the original zip file automatically for you, leaving a folder with a bunch of files in it at the top level of your Downloads folder with sort of a cryptic name, but it always starts with the author's name, so you shouldn't have any trouble finding it. So no need to worry about zip files, the Mac will take care of it, unlike how Windows apparently works (i.e. much easier).
Next, insert your SD card that you formatted on the Stream into your Mac's SD card reader slot and you'll see it show up on your desktop alongside your hard drive icon. Open it up and you'll see several folders, the first of which is named $VRDTB, which is the folder you want to open, since it's where you want to copy your "digital talking books" (thus the "DTB" in the name of the folder).
Open up your Mac's Downloads folder, find the folder in there for the book you downloaded, click it, select Copy (or press Command-C), click on the $VRDTB window on your SD card and click Paste (or press Command-V). You'll see a copy in progress dialog appear, then it'll finish. Copy more books over as you wish.
Click the SD card's icon on your desktop now, and choose Eject Disk (or press Command-E), remove the SD card after a few seconds during which it will vanish from your desktop, and reinsert the SD card into your Stream.
You should now be able to navigate to your book title on the Stream, with your books you just copied being included in the "Talking Books" directory/folder on your Stream.
Enjoy!